The >64GB memory kits are all ECC memory, which is a different thing to standard memory. Ryzen chips unofficially support it, but Intel don't. I'd guess (and emphasis on "guess") that it's not worth PCS stocking 128GB kits of memory for the very rare customer who needs it. For understandable reasons, they only sell memory as it comes in kits from the manufacturer, rather than combining multiple kits in builds.